This invention relates to distance-measuring equipment, and in particular, to interferometers.
In the fabrication of an integrated circuit, there are steps during which a wafer rests on a moveable stage under a projector. As the stage moves, it experiences linear translation in a direction parallel to one of two coordinate axes that define the plane of the wafer. In addition, the stage experiences rotation about any of the three coordinate axes. During these steps, it is desirable to know, with great precision, the position and orientation of the moveable stage relative to the projector.
One approach to determining the motion of the stage is to use an interferometer to determine the distance to a measurement spot along the edge of the stage. This, however, only provides the extent to which the stage has translated along one of the coordinate axes. It does not provide information on the rotation of the stage about the three coordinate axes.
To obtain information about both the rotation and the translation of the stage, it is known to use three interferometers to measure the distance to three different reference spots that define a triangle on the edge of the stage. From these three distance measurements, one can infer the extent and direction of rotation experienced by the stage. For example, if a first reference spot is found to have moved closer to the interferometer and a second reference spot immediately below the first spot is found to have moved further from the interferometer, one can infer that the plate has rotated about a line joining the first and second reference spots. From the relative positions and distances to the first and second reference spots, one can calculate the extent of this rotation.
The invention provides a multi-axis interferometer in which constituent optical elements are supported by a single mounting block.
In one aspect, the multi-axis interferometer includes a mounting block and first and second polarizing beam-splitter cubes contacting first and second faces of the mounting block. A beam-distribution system, which is in optical communication with the first and second polarizing beam-splitter cubes, contacts a third face of the mounting block.
The multi-axis interferometer can also include one or more retro-reflectors. These retro-reflectors are in optical communication with the first polarizing beam-splitter cube, the second polarizing beam-splitter cube, or both. The retro-reflector can be a polarization preserving retro-reflector.
In some embodiments, the retro-reflectors optically contact the polarizing beam-splitter cubes. In other embodiments, they optically contact the mounting block.
Other embodiments of the interferometer are those in which a retarder optically contacts one or both of the first and second polarizing beam-splitter cubes. The retarder includes first and second substrates separated by a gap, with an adhesive material filling that gap. A birefringent polymer is embedded in the adhesive material. The adhesive material and the birefringent polymer are selected to be index matched to each other.
The mounting block can be a solid block. However, some embodiments of the invention include a mounting block having walls forming one or more tunnels extending between first and second faces thereof. Some of these tunnels can be disposed to permit passage of a light beam between the beam-distribution system and one of the first and second polarizing beam-splitter cubes.
In another aspect, the invention includes a method of obtaining an interferometric measurement by optically contacting a first polarizing beam splitter cube to a first face of a mounting block; optically contacting a second polarizing beam splitter cube to a second face of a mounting block; and optically contacting a beam-distribution system to a third face of the mounting block, thereby placing the beam-distribution system in optical communication with the first polarizing beam-splitter cube and the second polarizing beam-splitter cube.
In other embodiments, first and second sub-aperture coatings are placed on a first face of a first prism in the beam delivery system. A second face of a second prism optically contacts the first and second sub-aperture coatings.
An interferometer system according to the invention can have one or more of the following advantages.
The mounting block absorbs most of the stresses associated with mounting the interferometer to a base plate and avoids transmitting those stresses to the other components of the interferometer.
The optional tunnels in the mounting block reduce the path length traversed by the beams in glass, thereby avoiding undesired distortions resulting from imperfections in the glass.
Optical contacting of prisms in the beam delivery system reduces reflections and wavefront distortions resulting from the crossing of boundaries between transmission media.
Polymer retarders reduce stray reflections resulting from using conventional quartz wave plates.
These and other features of the invention will be apparent from the following detailed description and the accompanying figures, in which: